A. Field of the Invention
This invention relates to the field of non-destructive evaluation of stress in materials, and more particularly to the use of elastic waves, sometimes called ultrasonic waves, for measuring stress.
B. Description of the Prior Art
The only well-developed method of nondestructively measuring stress in materials is the x-ray diffraction method. This method depends upon the measurement of distances between planes of atoms by exposing the material to x-rays and measuring the diffraction of the x-rays. Application of this method is severely restricted by the fundamental inability of x-rays to probe deeper than about a thousand atomic layers into the material, and by the method's total inapplicability to non-crystalline materials.
Other possible methods of measuring stress utilizing ultrasonics, electromagnetics, or nuclear hyperfine effects are in a very early stage of development. The present invention is limited to ultrasonic methods of measuring stress. All ultrasonic methods depend in principle upon the fact that the velocity of propagation of ultrasound (elastic waves) in a solid medium is influenced by the state of strain of the medium. Although the effect is small, its detection and measurement are within the present state of the ultrasonic art.
However, the velocity of sound is also affected by numerous other factors related to the condition of the material such as its microstructure, heat treatment, grain orientation, density, and homogeneity. Therefore, the determination of the absolute velocity of sound in a material does not give an accurate indication of stress in the material unless standards which accurately represent all the other velocity-affecting conditions are available. To overcome this problem with absolute velocity determination, a known technique called shear wave birefringence is used. See, for example, "Shear Wave Birefringence," by N. N. Hsu, in Proceedings of a Workshop on Nondestructive Evaluation of Residual Stress, Aug. 13-14, 1975, published by NTIAC, San Antonio, Texas. This technique is based upon measuring the difference in velocity of piezoelectrically generated shear waves which are orthongonally polarized by the anisotropic stress existing within the material. According to this technique, only the difference in velocity between two shear waves is measured. Since this difference in velocity is caused primarily by the difference in stress in two orthogonal directions within the material, the effect of material condition can be minimized or at least accounted for.
According to the prior art, piezoelectric transducers are required to inject ultrasonic waves into the material being measured. These transducers utilize an oriented crystal which is strained along a particular crystallographic axis in response to an electric field applied to the crystal (the piezoelectric effect). Consequently, the piezoelectric transducer must be rigidly attached or coupled by a very viscous fluid or a solid bond to the material being evaluated in order to inject an ultrasonic wave into the material. Additionally, since motion of the piezoelectric is generated only along a specific crystallographic axis, single piezoelectric crystals can create a driving force on the surface of the material in only a single direction as defined by the crystal orientation.
If a uniaxial stress exists in the material being measured, and if a prior art piezoelectric transducer is oriented parallel or transverse to the stress; then the transverse wave generated by the transducer will not be polarized (or decomposed) into two separate, orthogonally polarized shear waves. Rather, only a single transverse wave oriented in the same direction as the transducer will be created. To generate a second wave having a different velocity, the transducer can be rotated 90.degree. to obtain a second wave at 90.degree. to the first wave; or the transducer can be rotated less than 90.degree. in order to create two orthogonally polarized waves as discussed earlier. Because the piezoelectric transducer must be in rigid contact or otherwise physically coupled to the material, rotation of the transducer to obtain a second wave or a pair of orthogonally polarized shear waves of measurable magnitude is very inconvenient.